1. Field of the Invention
The present invention relates in general to display systems, and in particular, to a method and apparatus for providing image distortion correction for a multi-scan display.
2. Description of the Related Art
Cathode ray tubes ("CRT") are typically employed in electronic display systems, such as a television receiver or a display apparatus including a CRT numerical display. Each color CRT consists of three electron guns (a red, a white and a blue electron gun) and a phosphor screen that is located inside an evacuated glass envelope. Each electron gun generates a beam of electrons that is accelerated towards the screen by a positive anode voltage. An image is formed on the screen according to the density of the projected electron beam. The electron beam is also deflected by the electron magnetic field formed by the deflection coil so that the deflection angle of the electron beam may be deformed if the electromagnetic field is altered due to imperfections in the deflection yoke, misalignment and/or misadjustment of the deflection yoke.
Such imperfections, misalignment and or misadjustment of the deflection yoke typically results in providing a distorted image, as shown in FIGS. 1B, 1C, 1D, 1E, and/or 1F instead of providing the desired image shown in FIG. 1A.
Imperfections in the image due to horizontal deflection appear on the left and right sides of the image resulting in non-rectangularity of the images (e.g. due to Keystone distortion--see FIG. 1D, or due to horizontal pin distortion,--see FIGS. 1E and 1F etc.). These are corrected by adding a vertical frequency component to the horizontal deflection current and therefore varying the deflection current angle at the top, middle and bottom portions of the screen. This type of correction is currently used in most display monitors, including multi-scan and single-scan monitors. To correct for imperfections at the top and bottom sides of the screen, manual adjustment of the deflection yoke is commonly used. Sometimes, a magnetic field (e.g. a static magnet) can be added in the vertical deflection circuit. This will change the angle of the vertical deflection current at the left, middle and right sides of the screen. This technique has been used successfully in single-scan monitors, which are typically used in convention television sets. Unfortunately, the technique provides image distortion correction over a very narrow range of frequencies, and therefore, cannot be implemented in multi-scan monitors.
Accordingly, there is a need in the technology for method and apparatus for providing image distortion correction for a multi-scan display.